The use of membranes to effect separation of gas/gas, liquid/liquid, and liquid/solid mixtures and solutions has achieved general industrial applicability by various methods, among them being ultrafiltration, hyperfiltration, reverse osmosis, and dialysis. In general, membrane elements associated with these processes are contained in vessels called modules, comprising a shell having membranes arranged as to permit the introduction of a feed stream on the upstream face of the membranes, means for collecting permeate which passes through the membranes and emerges on their downstream faces, and means for keeping feed and permeate materials from commingling. Commonly assigned U.S. Pat. No. 4,207,192 discloses a hollow filament separatory module and method of fabrication, and is incorporated by reference herein.
In one method of forming a hollow fiber separatory element successive layers of fiber are wound upon one another in continuous helices of opposite hand eventually creating an annular bundle. The fiber assembly used as the principal example in the present disclosure for illustrative but not limiting purposes can be described as a relatively thick-walled annulus of a generally constant cross-section. Other methods of assembling hollow fiber bundles are known and may be contemplated as encompassed by the present invention. Likewise, certain features of the invention to be described are applicable to tubular, flat-sheet and spiral membrane configurations, and are contemplated as subjects of this invention.
Membrane assemblies are useful as pressure-driven separatory elements for multicomponent fluid feeds only if active areas of membrane are effectively sealed inside a pressure vessel by suitable means to prevent the commingling of pressurized feed fluid with the permeate fluid which passes through the membranes and emerges on the downstream side of their separation-functioning surfaces.
In the case of hollow fiber membrane elements, one approach to achieving the sealing-off of the pressurized feed relies on providing that one or both ends of the originally as-wound bundle is (are) encapsulated in a potting compound. This may be achieved for example by immersing the bundle end(s) in a polymerizable material in a mold. The outside diameter of the bundle may be just slightly smaller than the inside diameter of the mold. The result is to provide, eventually after suitable curing of the polymerizable material, a massive polymerized cylindrical plug encapsulating one or both ends of the bundle.
Fibers encapsulated in the plug are severed in such a way as to expose open fiber bores (See referenced U.S. Pat. No. 4,207,192). The cut ends of fibers, out of whose bores permeated fluid emerges during the separation process must be kept sealed away from the feed fluid acting on the fibers in the pressurized zone. This sealing may be conveniently achieved by the installation of "O" rings which act cooperatively between the outer surface of the potting plug and the inner surface of the pressure vessel containing the bundle.
In view of the fact that the outside diameter of the plug was formed larger than the outside diameter of the bundle, and there must be an "O" ring space between the cylindrical surface of the plug and the inside surface of the pressure vessel, it follows that there will be some, perhaps considerable, clearance between the surface of the bundle in the region of its active length and the inside of the pressure vessel is each of these is of a constant diameter. One expedient to overcome the excess clearance is to make the pressure shell with two diameters - one to accomodate the potted bundle end with "O" rings, the other to conform appropriately to the bundle diameter.